Dissolved and solid-phase speciation of Mn and Fe was measured in the porewaters of sediments recovered from three sites in the Greater St. Lawrence Estuary: the Saguenay Fjord, the Lower St. Lawrence Estuary (LSLE) and the Gulf of St. Lawrence (GSL). At all sites and most depths, metal organic ligand complexes (Mn(III)–L and Fe(III)–L) dominated the sedimentary porewater speciation, making up to 100% of the total dissolved Mn or Fe. We propose that these complexes play a previously underestimated role in maintaining oxidized soluble metal species in sedimentary systems and in stabilizing organic matter in the form of soluble metal–organic complexes. In the fjord porewaters, strong (log K_COND > 13.2) and weak (log K_COND < 13.2) Mn(III)–L complexes were detected, whereas only weak Mn(III)–L complexes were detected at the pelagic and hemipelagic sites of the GSL and LSLE, respectively. At the fjord site, Mn(III)–L complexes were kinetically stabilized against reduction by Fe(II), even when Fe(II) concentrations were as high as 57 μM. Only dissolved Mn(II) was released from the sediments to overlying waters, suggesting that Mn(III) may be preferentially oxidized by sedimentary microbes at or near the sediment–water interface. We calculated the dissolved Mn(II) fluxes from the sediments to the overlying waters to be 0.24 µmol cm⁻² year⁻¹ at the pelagic site (GSL), 11 µmol cm⁻² year⁻¹ at the hemipelagic site (LSLE) and 2.0 µmol cm⁻² year⁻¹ in the fjord. The higher benthic flux in the LSLE reflects the lower oxygen concentrations (dO₂) of the bottom waters and sediments at this site, which favor the reductive dissolution of Mn oxides as well as the decrease in the oxidation rate of dissolved Mn(II) diffusing through the oxic layer of the sediment and its release to the overlying water.

中文翻译：

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河口沉积物中锰和铁的形态和迁移率

在大圣劳伦斯河口的三个地点：萨格奈峡湾，圣劳伦斯河下游河口（LSLE）和圣劳伦斯湾（以下地区）回收的沉积物的孔隙水中测量了锰和铁的溶解相和固相形态GSL）。在所有位置和最深处，金属有机配体络合物（Mn（III）–L和Fe（III）–L）主导着沉积孔隙水形态，占溶解的Mn或Fe总量的100％。我们建议这些络合物在维持沉积系统中氧化的可溶性金属种类以及以可溶性金属-有机络合物的形式稳定有机物方面起着被低估的作用。在峡湾孔隙水中，强（log K _COND  > 13.2）和弱（log K _COND） <13.2）检测到Mn（III）-L复合物，而在GSL和LSLE的中上层和中上层站点分别仅检测到弱的Mn（III）-L复合物。在峡湾处，即使Fe（II）的浓度高达57μM，Mn（III）-L络合物也能被动力学稳定地防止被Fe（II）还原。仅溶解的Mn（II）从沉积物中释放到上覆水域，这表明Mn（III）可能被沉积物-水界面处或附近的沉积微生物优先氧化。我们计算出从沉积物到上层水体的溶解的Mn（II）通量在浮游点（GSL）为0.24 µmol cm - ² 年^-1，在半浮游点（LSLE）为11μmolcm - ² 年^-1和2.0 µmol厘米^-2 - ¹年在峡湾。LSLE中较高的底流通量反映了该位置底部水和沉积物的较低氧浓度（dO ₂），这有利于Mn氧化物的还原溶解以及溶解的Mn（II）扩散的氧化速率降低通过沉积物的含氧层并将其释放到上覆水中。